Our long term objective is to elucidate the mechanism of activation and suppression of cytolytic T-lymphocytes, and thereby find methods of inducing and suppressing cellular immunity. As intermediate objectives we have set two goals, first to determine the relative roles of fixed tissue cells and mobile leucocytes in the process of stimulation and second, to determine the relative roles of macrophages and other leucocytes. The first question is justified by the finding that cultured allografts may survive longer than controls and the second by the complementary findings that the number of monocytes and macrophages in the body is much higher and the number required for stimulation much lower than originally thought possible. Thus, the reason for the requirement for phagocytic cells in in vitro reactions takes on particular cogency. Short term objectives: 1. Determine whether the prolongation of allograft survival produced by gamma-radiation of donor is due to an effect on the tissue cells of the graft or to loss of passenger leucocytes through leucopenia. 2. Determine the effect of prolonged in vivo "culture" of thyroid and skin allografts in a T-cell depleted recipient (intermediate recipient) on survival of the allograft after retransplantation to an untreated recipient syngeneic to the intermediate recipient. 3. Compare the survival of allografts placed on chimeric recipients across major histocompatibility barriers with those made across minor histocompatibility barriers. 4. Determine if resting lymphocytes depleted of all monocytes are capable of activating a cytolytic response in allogeneic lymph node cells. 5. Determine the effect of helper cells in either stimulating or responding populations on the ability of parental LNL to respond to resting and formalin treated LNL. 6. Determine the stimulating ability for allogeneic lymphocytes of selected mouse tumors.